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LECTURE
Frequency Shift Keying (FSK) o Frequency of signal is varied to represent binary 1 or 0 o The frequency of the signal during each bit duration is constant and depends on the bit (0 or 1) o Both peak amplitude and phase remains constant
Effect of Noise on FSK o Avoids most of the Noise problems of ASK o Because Rx device is looking for specific frequency changes over a given number of periods, it can ignore voltage spikes o The limiting factors of FSK are the physical capabilities of the carrier
BW of FSK
o Although FSK shifts between two carrier frequencies, it is easier to analyze as two co-existing frequencies o BW required for FSK is equal to the Baud rate of the signal plus the frequency shift o Frequency Shift=Difference b/w two carrier frequencies o BW= (fc1 – fc0) +Nbaud
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Example 5. Find the minimum BW for an FSK signal transmitted at 2000 bps. TX is in half duplex mode and carrier must be separated by 3000 Hz
Solution : For FSK, if fc1 and fc2 are the carrier frequencies, then: BW=Baud Rate + (fc1 – fc0) Baud rate is the same as bit rate BW=2000 + (fc1 – fc0) = 2000 + 3000 = 5000 Hz
Phase Shift Keying (PSK)
o In PSK, phase of carrier is varied to represent binary 1 or 0 o Both peak amplitude and frequency remains constant as the phase changes o For Example: if we start with a phase of 0 degrees to represent binary 0 , then we can change the phase to 180 degrees to send binary 1 o The phase of signal during duration is constant and its value depends upon the bit (0 or
2PSK The above method is often called 2 PSK, or Binary PSK, because two different phases ( 0 and 180 degrees) are used
Figure makes this point clear by showing the relationship of phase to bit value A second diagram called constellation diagram or phase state diagram shows same relationship by illustrating only the phases Effect of Noise on PSK o PSK is not susceptible to the noise degradation that affects ASK, nor to the bandwidth limitations of FSK o Smaller variations in signal can be detected reliably by the receiver
4PSK docsity.com
QAM
- Limitations of PSK:
- PSK is limited by the ability of the equipment to distinguish small differences in phase
- This factor limits its potential bit rate
- So far we have been changing only of the characteristics of the sine wave, But what if we alter two
o What should these two characteristics be? o BW limitations make combination of FSK with other changes practically useless o Why not combine ASK and PSK? o ‘x’ variation in phase and ‘y variations in amplitude result into a total of x * y variations and corresponding no. of bits per variation
Quadrature Amplitude Modulation (QAM)
Variation of QAM o Variations of QAM are numerous o Any measurable amount changes in amplitude can be combined with any measurable no. of changes in Phase
4 QAM & 8 QAM (Figure) o In both case no. of amplitude shifts is more than the no. of phase shifts o Because amplitude changes are susceptible to Noise , number of phase shifts used by QAM is always larger than the amplitude shifts
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Time domain plot of 8 QAM
Three possible variations of 16 QAM
Bandwidth for QAM o BW required for QAM is the same as in the case of ASK and PSK o QAM has the same advantages as PSK over ASK o Bit Baud Comparison
Bit Baud Comparison Consult book section 5.
Example 5. A constellation diagram consists of eight equally spaced points on a circle. If bit rate is 4800 bps, what is the Baud Rate?
Solution : Constellation indicates 8 PSK with the points 45 degree apart Baud Rate= 4800 / 3 = 1600 baud
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